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nonadditive components

It is unique strictly not the additive contribution in (6.8) and (6.9) is the summand 4RT (for linear molecules 7/2 RT).eta the size falls into forward and vrashchatelyonym (external) degrees of freedom, characterises a molecule as whole and is constant for all molecules, irrespective of a structure and composition.

The share 4RTв initial (eksperimentalyonyh) partial contributions of fragments in ΔfHо for each molecule training mnozheyostva is equal 4RT, where n - quantity of the fragments making a molecule. Therefore for n

Different connections these shares, first, are various and, secondly, depend on a way razyobienija. To result a share of the yielded size for concrete Bensonovsky group after payorametrizatsii it is not obviously possible owing to the procedure of the parametrization made routinely by a least square method. Thus, if "initial" components ΔfHoвзяты from the training set consisting from small and umerenyono of complicated molecules (usual practice) the share 4RTво contributions of groups received from them will be significant and in total theoretical value ΔfHoисследуемой the big molecule forward and rotary contributions, most likely, are not equal 4RT. It is the reason of occurrence of a methodical error of the additive models when «external degrees of freedom are considered as internal».

Consideration ΔfHoчленов one homologous series shows, that for small molecules theoretically calculated value ΔfHoбудет is underestimated and overestimated for bolyoshih (at the expense of summation of shares 4RTобразующих their groups), i.e. for higher homologues with growth of number of groups, forward and rotary contributions in ΔfHoмогут considerably to exceed 4RT.Наиболее preferable value Δfloдает calculation for molecules, comparable with basic connections by quantity of groups.

Separate discussion is demanded by temperature dependences of these contributions. The yielded size is directly proportional to absolute temperature and, for example, at T = 298 To the contribution not the additive summand 4RT (7/2 RT) makes 9.91 (8.67) kdzh/MOLE, and at 1000 To 33.24 (29.09) kdzh/MOLE, that much more an experiment error, the best kvantovohimicheskih calculations and, especially, declared to "chemical accuracy» is already equal. The share carried on groups 4RT (7/2 RT) grows with temperature on the one hand, and with druyogoj in high-temperature areas experimental data small moyolekul at which the share specified above 4RTво contributions is great prevail. It will inevitably lead to corresponding growth of an error ΔfHoT, especially at definition of such sizes as pressure of cycles, not valent interactions etc. In other words, the share pogreshnoyosti the forward and rotary contribution to the general error also grows directly
To proportionally absolute temperature also can become prevailing at heats.

6.2.3.

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A source: Turovtsev Vladimir Vladimirovich. Creation and application of a quantum mechanical model for calculating the thermodynamic properties of substances in a wide range of temperatures. Thesis for the degree of doctor of physico-mathematical sciences. Tver - 2014. 2014

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